A “six week” embryo, about the size of a pea.Photo: Yiming Chen (Getty Images)

In the wake of a Georgia law banning abortions after five to six weeks of pregnancy, a few people—I’m not naming names—have shown that they have no idea how weeks are counted in pregnancy. Folks, on the day you have sex to make a baby, you are considered two weeks pregnant already. And you can’t possibly even know that you’re pregnant until four or five weeks.

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That’s because gestational age (weeks of pregnancy) is not how long you’ve had an embryo or fetus in your body, but how long it’s been since the first day of your last period. Let’s walk through this, using a textbook example of a woman (let’s call her Jane) with a textbook-perfect cycle. She starts her period on January 1 and has sex only once in her life, resulting in a pregnancy. These dates would vary from person to person, so this is just an example:

Jan 1: Jane’s period starts. She is shedding the lining of her uterus. She is definitely not pregnant.

January 12 or so: She has sex.

January 14: She ovulates (releasing an egg cell from her ovary). Shortly afterward, the egg meets one of the sperm cells from a few days ago.

January 21: The developing embryo implants in her uterus. She is now pregnant—specifically, three weeks pregnant.

January 28: Levels of pregnancy hormones are high enough that maybe, just maybe, a very sensitive pregnancy test could detect them.

January 30: Jane, who is watching the calendar like a hawk, notices her period is late. She is more than four weeks pregnant.

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So the earliest you can possibly learn that you’re pregnant is at a gestational age of “four weeks”—which means the embryo has been implanted in your uterus for only one actual week, and has only existed for two weeks. But the dates are counted as if the pregnancy began on January 1.

This is an optimistic example, for many reasons. For example, most people’s periods don’t come on a textbook 28-day cycle. If your period tends to come anywhere between, say, 29 to 34 days, then you won’t start wondering until a bit later. And this assumes that you’re watching the calendar; if you aren’t expecting to get pregnant, you might not realize you’re late until a week or two have passed. The American Pregnancy Association notes that most women find out they are pregnant at four to seven weeks gestational age (which is two to five weeks after having sex that results in conception).

A missed period isn’t a great pregnancy signal for everyone, either. Some people have irregular periods, especially if they are using a form of birth control that lets them skip their period, or gives them lighter periods. Meanwhile, it’s possible to have bleeding around that three-week mark when the embryo implants, and you might think “hm, my period is a little lighter and earlier than usual,” but not really worry about it. There are also medical reasons why a person might miss their period for reasons other than pregnancy.

So, what about pregnancy symptoms? Morning sickness, for example, doesn’t tend to start showing up until weeks five to seven, and some people don’t get morning sickness at all.

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If you go to a doctor or clinic for a pregnancy test, then we’re adding more time. Say you think your period might be a week late, and you find your way to the clinic a few days later. You could easily be at six weeks before you get a test at all.